1. Field of the Invention
This invention relates to a matrix optical switch for switching the transmission path of an optical signal by connecting plural input sides to plural output sides, respectively, and particularly to a matrix optical switch that can switch the transmission path of an optical signal without requiring a driving unit for each optical switch.
2. Description of the Related Art
The current communication networks such as LANs (local area networks) and WANs (wide area networks) usually employ a communication system that transmits information on electrical signals.
A communication method of transmitting information on optical signals is used only in trunk networks for transmitting a large quantity of data and some other networks. These networks use “point-to-point” communications and have not yet developed into communication networks that can be called “photonic networks”.
To realize such a “photonic network”, an “optical router”, an “optical switching hub” and the like that have functions similar to the functions of devices such as a router and a switching hub for switching the destination of an electrical signal are needed.
Particularly for the optical router used in trunk networks, a matrix optical switch of N×N (multiple-by-multiple) type is important, as in the current router of electrical signals. There is an optical switch having an optical waveguide formed in a semiconductor, to which carriers are injected to change the refractive index and thus switch the transmission path of an optical signal.
The following are references of the related art of the conventional matrix optical switch that switches the transmission path of an optical signal by connecting plural optical waveguides on the input side to plural optical waveguides on the output side, respectively:
JP-A-6-75179;
JP-A-8-163031;
JP-A-9-105959;
JP-A-10-308961; and
Hiroaki Inoue et al., “An 8 mm Length Nonblocking 4×4 Optical Switch Array,” IEEE JOURNAL ON SELECTED AREA IN COMMUNICATIONS, Vol. 6, No. 7, p. 1262–1266 (1988).
FIG. 1 is a plan view showing an example of a part of the conventional matrix optical switch described in Hiroaki Inoue et al.
In FIG. 1, an optical waveguide 2 and an optical waveguide 3 are formed on a semiconductor substrate 1, intersecting each other. An optical waveguide 4 is formed near the intersection of the optical waveguide 2 and the optical waveguide 3 so as to connect the input side of the optical waveguide 3 to the output side of the optical waveguide 2. In carrier injection areas indicated by “IR01” and “IR02” in FIG. 1, electrodes (not shown) for injecting carriers are provided.
In the actual matrix optical switch, plural stand-along optical switches as shown in FIG. 1 are arranged to form an N×N (multiple-by-multiple) matrix optical switch.
The operation in the conventional example shown in FIG. 1 will now be described with reference to FIGS. 2 and 3. FIGS. 2 and 3 are explanatory views for explaining the operation in the conventional example shown in FIG. 1. In FIGS. 2 and 3, the semiconductor substrate 1, the optical waveguide 2, the optical waveguide 3, the optical waveguide 4 and the other elements are denoted by the same numerals and symbols as in FIG. 1.
When the optical switch shown in FIG. 1 is off, no currents are supplied from the electrodes (not shown) to the carrier injection areas indicated by “IR01” and “IR02” in FIG. 1.
Therefore, the refractive index does not change in the carrier injection areas indicated by “IR01” and “IR02” in FIG. 1. For example, as indicated by “PS11” in FIG. 2, an optical signal becomes incident from an incidence end indicated by “PI01” in FIG. 2, then travels straight through the intersection and is emitted from an emission end indicated by “PO01” in FIG. 2.
On the other hand, when the optical switch shown in FIG. 1 is on, electrons and holes are injected from the electrodes (not shown). Therefore, carriers (electrons and holes) are injected into the carrier injection areas indicated by “IR01” and “IR02” in FIG. 1.
As the refractive index is lowered in the carrier injection areas indicated by “IR01” and “IR02” in FIG. 3 by a plasma effect, for example, an optical signal indicated by “PS21” in FIG. 3 is totally reflected by the low refractive index part generated in the carrier injection area indicated by “IR01” in FIG. 3 and propagates through the optical waveguide 4 as indicated by “PS22” in FIG. 3.
Then, the optical signal indicated by “PS22” in FIG. 3 is totally reflected by the low refractive index part generated in the carrier injection area indicated by “IR02” in FIG. 3, then propagates through the optical waveguide 2 as indicated by “PS23” in FIG. 3, and is emitted from an emission end indicated by “PO02” in FIG. 3.
As a result, an N×N (multiple-by-multiple) matrix optical switch can be constructed by arranging plural optical switches each of which has a bypass-like optical waveguide formed near the intersection of two optical waveguides 2 and 3 intersecting each other so as to connect the input side of the one optical waveguide to the output side of the other optical waveguide and which propagates an optical signal through the bypass-like optical waveguide to switch the propagation path when the optical switch is on.
However, in the matrix optical switch having the plural conventional optical switches shown in FIG. 1, the following relational expression holds:nc=ni×no−1  (1)where “nc” represents the number of intersections at which the propagation path of an optical signal must be switched, “ni” represents the number of optical waveguides on the input side, and “no” represents the number of optical waveguides on the output side. Therefore, the increase in the number of optical waveguides on the input side and output side increases raises a problem that a very large number of intersections exist at which the propagation path of an optical signal must be switched.
There is also a problem that it is difficult to realize a driving unit for independently driving (i.e., injecting carriers) the individual electrodes arranged at the very large number of intersections (i.e., the conventional optical switches as shown in FIG. 1).